We are interested in the genetics and regulation of virulence factors of gram negative pathogens, including Shigella and Vibrio cholerae. Members of the Shigella species are found both extracellularly and intracellularly within the host, and distinct sets of genes are expressed in each of these environments. We are studying bacterial gene expression in vivo by a variety of molecular and microscopic techniques, and we have identified genes that are required for growth in the intracellular environment of the host. Bacterial genes found to be important for growth and survival in vivo include those that encode components of iron transport systems, metabolic pathways and cell surface proteins.